1. Field of the Invention
The present invention relates to a hydraulic four-wheel driving articulated vehicle having mutually rotatably connected front and rear frames, wherein a hydraulic motor for driving right and left front wheels is mounted on the front frame, a hydraulic motor for driving right and left rear wheels is mounted on the rear frame, and the hydraulic motors are fluidly connected in series to a hydraulic pump. Especially, the present invention relates to construction for smoothing the turning operation of a vehicle body by rotating the front and rear frames while the vehicle is stationary, that is, the operation of so-called “stationary steering”.
2. Related Art
Conventionally, there is a well-known articulated vehicle having mutually rotatably connected front and rear frames 11 and 21 supporting respective transaxles 10 and 20 as shown in FIGS. 1 and 2, and as disclosed in the Japanese Patent Laid Open Gazette 2000-270651, for example.
To steer the articulated vehicle, the front and rear frames are rotated relative to each other so that the vehicle body is bent in a dogleg-shape when viewed in plan. As shown in FIGS. 2 and 4, the vehicle body is bent by expanding/contracting a piston rod 17a of a power steering cylinder 17 interposed between the front and rear frames 11 and 21. Pressure oil supplied to the power steering cylinder 17 is controlled by a power steering cylinder driving hydraulic circuit 58 including a directive control valve interlocking with a steering wheel 4.
The articulated working vehicle is designed so that its four wheels are hydraulically driven. For example, as shown in FIGS. 2,3 and 5, the transaxle 10 on the front frame 11 incorporates a hydraulic motor 31m for driving front axles 12L and 12R, i.e., front wheels 13L and 13R, and the transaxle 20 on the rear frame 21 incorporates a hydraulic motor 41m for driving rear axles 22L and 22R, i.e., rear wheels 23L and 23R. A hydraulic pump unit 50 incorporates a hydraulic pump 50p for supplying pressure oil to the transaxles 10 and 20, thereby driving the front and rear axles 12L, 12R, 22L and 22R.
The hydraulic motors 31m and 41m in the respective transaxles 10 and 20 are fluidly connected to the hydraulic pump 50p in the hydraulic pump unit 50 in series so as to constitute a hydraulic circuit 27, as show in FIG. 5, so that the two hydraulic motors 31m and 41m are driven by one hydraulic pump 50p. Such a hydraulic circuit system having only one hydraulic pump is suitable for hydraulically driving four wheels of the articulated working vehicle.
It is now assumed that a drive-mode switching valve 28 is opened to fluidly connect the hydraulic motor 31m to the hydraulic pump 50p, and during forward traveling of the vehicle, as shown in FIG. 5, the hydraulic pump 50p discharges pressure oil into a passage 27b connected to the hydraulic motor 41m so that the oil circulating in the hydraulic circuit 27 is directed, as outlined by arrows 24L, so as to be supplied to the hydraulic motor 41m and then to the hydraulic motor 31m. A charge pump 53p supplies hydraulic oil into the hydraulic circuit 27 through charge valves 53a and 53b. 
When the vehicle body is bent by rotating the steering wheel 4 for turning of the vehicle when the vehicle is stationary, that is, by the stationary steering operation, each of the four wheels are moved as shown in FIG. 6.
In FIG. 6, the right front wheel 13R is rotated in the direction of an arrow 114R by a resistance force from the ground surface so as to travel forward for a distance 115R. On the other hand, the left front wheel 13L is rotated in the direction of an arrow 114L, opposite to the arrow 114R by a resistance force from the ground surface so as to travel backward for a distance 115L, which is longer than the distance 115R traveled by the right front wheel 13R. As a result, the front wheels 13L and 13R travel backward for a distance equaling the difference between the distances 115L and 115R. Due to the backward traveling of the front wheels 13L and 13R, the hydraulic motor 31m rotates in the backward traveling direction and acts as a pump such as to discharge backpressure oil not into a passage 27a connected to the hydraulic pump 50p, but into a passage 27c connected to the hydraulic motor 41m. 
Simultaneously, as shown in FIG. 6, the right rear wheel 23R is rotated in the direction of an arrow 124R by a resistance force from the ground surface so as to travel backward for a distance 125R. On the other hand, the left rear wheel 23L is rotated in the direction of an arrow 124L, opposite to the arrow 124R by a resistance force from the ground surface so as to travel forward for a distance 125L, which is longer than the distance 125R traveled by the right rear wheel 23R. As a result, the rear wheels 23L and 23R travels forward for a distance equaling the difference between the distances 125L and 125R. Due to the forward traveling of the rear wheels 23L and 23R, the hydraulic motor 41m rotates in the forward traveling direction and acts as a pump such as to discharge backpressure oil into the passage 27c connected to the hydraulic motor 31m. 
Namely, as shown in FIG. 5, the hydraulic motors 31m and 41m discharge backpressure oil toward each other along respective arrows 25a and 25b, so that the backpressure oil is confined and excessively accumulated in the passage 27c interposed between the hydraulic motors 31m and 41m, thereby restricting rotation of the hydraulic motors 31m and 41m and rotation of the four wheels. This is the reason why the conventional hydraulic four-wheel driving articulated vehicle cannot be smoothly steered when it is stationary, i.e., the stationary steering of the vehicle cannot be performed smoothly.